The device of this invention is designed for use in a bar tacking sewing machine. Bar tacking is the term used to describe the sewing of small stitch patterns which are generally used for reinforcing joints in shoes and other garments. These patterns are generally limited to a specific number of stitches in the range of from 10 to 100 stitches per pattern and cover only a small area of the workpiece. The operation is performed by moving the workpiece under the needle and this motion is achieved automatically by means of a work clamp which is mounted for movement along two axes relative to the needle. Work clamp movement is controlled by a style or feed cam which is operatively linked to the clamp. The style cam is generally driven by means of a shaft connected to the main needle bar drive shaft through a gear train. Thread cutting is controlled by a second cam connected to the same shaft but mounted opposite to the style cam.
Generally, the work clamp of this type of machine consists of a mechanically operated spring biased device which squeezes the workpiece between upper and lower clamping elements. The lower element is usually fixed in position with the upper element being forced downward by a spring which may be released by a lever associated with the thread cutting operation. The clamping elements have openings or windows which allow access to the workpiece by the sewing head. The pattern which is to be sewn is stitched within this opening. The upper clamping element may be split into a pair of separately releasable members to facilitate the clamping and registration of a workpiece which consists of multiple parts.
By operating the clamping mechanism in association with the thread cutter through a cam, a very rigid sequence of events are forced into the sewing operation, namely, the workpiece will be unclamped immediately after the thread is cut without exception. This partially defeats the flexibility which is desired on automatically controlled machines where workpiece movement and collateral operations are under electronic control. In addition, where a dual element clamp is used, only a limited amount of separate movement can be achieved.
Specifically, in those instances where it is desired to sew several distinct designs in one pattern, it is possible that the thread will be cut repeatedly after each design. Unclamping the workpiece after each design would result in a loss of registration of the parts and this would require costly operator involvement and loss of time.
The clamp of this invention, therefore, releases the mechanism from the control of the knife actuating lever and provides separate pneumatic actuators which may be controlled automatically. For further ease of use quick change fittings are provided on the upper and lower clamping elements to allow fast removal and replacement when different patterns are to be sewn.